1. Field of the Invention
The present invention generally relates to antigen-specific immunoconjugates for selectively delivering effector moieties that influence cellular activity. In addition, the present invention relates to nucleic acid molecules encoding such immunoconjugates, and vectors and host cells comprising such nucleic acid molecules. The invention further relates to methods for producing the immunoconjugates of the invention, and to methods of using these immunoconjugates in the treatment of disease.
2. Background Art
The selective destruction of an individual cell or a specific cell type is often desirable in a variety of clinical settings. For example, it is a primary goal of cancer therapy to specifically destroy tumor cells, while leaving healthy cells and tissues intact and undamaged. A multitude of signal transduction pathways in the cell are linked to the cell's survival and/or death. Accordingly, the direct delivery of a pathway factor involved in cell survival or death can be used to contribute to the cell's maintenance or destruction.
Cytokines are cell signaling molecules that participate in regulation of the immune system. When used in cancer therapy, cytokines can act as immunomodulatory agents that have anti-tumor effects and which can increase the immunogenicity of some types of tumors. However, rapid blood clearance and lack of tumor specificity require systemic administration of high doses of the cytokine in order to achieve a concentration of the cytokine at the tumor site sufficient to activate an immune response or have an anti-tumor effect. These high levels of systemic cytokine can lead to severe toxicity and adverse reactions.
One way to deliver a signal transduction pathway factor, such as a cytokine, to a specific site in vivo (e.g., a tumor or tumor microenvironment) is to conjugate the factor to an immunoglobulin specific for the site. Early strategies aimed at delivering signal transduction pathway factors, such as cytokines, to a specific site in vivo included immunoglobulin heavy chains conjugated to various cytokines, including lymphotoxin, tumor necrosis factor-α (TNF-α), interleukin-2 (IL-2), and granulocyte macrophage-colony stimulating factor (GM-CSF). The immunoglobulin heavy chains were either chemically conjugated to a cytokine or the immunogobulin-cytokine conjugate was expressed as a fusion protein. See Nakamura K. and Kubo, A. Cancer Supplement 80:2650-2655 (1997); Jun, L. et al., Chin. Med. J. 113:151-153 (2000); and Becker J. C., et al., Proc. Natl. Acad. Sci. USA 93:7826-7831 (1996). Researchers observed that, not only were they able to target cytokines to specific sites in vivo, they were also able to take advantage of the fact that monoclonal antibodies have longer serum half-lives than most other proteins. Due to the systemic toxicity associated with high doses of certain unconjugated cytokines, i.e. IL-2, the ability of an immunoglobulin-cytokine fusion protein to maximize immunostimulatory activities at the site of a tumor whilst keeping systemic side effects to a minimum at a lower dose led researchers to believe that immunoglobulin-cytokine immunoconjugates were optimal therapeutic agents. However, one of the major limitations in the clinical utility of immunoglobulins as delivery agents is their inadequate uptake and poor distribution in tumors, partially due to the large size of the immunoglobulin molecule. See Xiang, J. et al., Immunol. Cell Biol. 72:275-285 (1994). Additionally, it has been suggested that immunoglobulin-cytokine immunoconjugates can activate complement and interact with Fc receptors. This inherent immunoglobulin feature has been viewed unfavorably because therapeutic immunoconjugates may be targeted to cells expressing Fc receptors rather than the preferred antigen-bearing cells.
One approach to overcoming these problems is the use of engineered immunoglobulin fragments. Numerous studies have detailed the characteristics of immunoglobulin fragment-cytokine immunoconjugates. See Savage, P. et al., Br. J. Cancer 67:304-310 (1993); Harvill, E. T. and Morrison S. L., Immunotechnol. 1:95-105 (1995); and Yang J. et al., Mol. Immunol. 32:873-881 (1995). In general, there are two common immunoglobulin fragment-cytokine fusion protein constructs, the F(ab′)2-cytokine expressed in mammalian cells and the scFv-cytokine expressed in Escherichia coli. See Xiang, J. Hum. Antibodies 9:23-36 (1999). Both the tumor-binding reactivity of the immunoglobulin parent molecule and the functional activity of the cytokine are maintained in most of these types of immunoconjugates. Recent preclinical studies have shown that these fusion proteins are able to target cytokines to tumors expressing the tumor-associated antigen in vivo, and to inhibit both primary and metastatic tumors in an immune competent animal model.
Examples of immunoglobulin fragment-cytokine immunoconjugates include the scFv-IL-2 immunoconjugate as set forth in PCT publication WO 2001/062298 A2; the immunoglobulin heavy chain fragment-GM-CSF immunoconjugate as set forth in U.S. Pat. No. 5,650,150; the immunoconjugate as set forth in PCT publication WO 2006/119897 A2, wherein scFv-IL-12 first subunit shares only disulfide bond(s) with IL-12 second subunit-scFv, and the immunoconjugate as described in PCT publication WO 99/29732 A2, wherein Ig heavy chain fragment-IL-12 first subunit shares only disulfide bond(s) with Ig heavy chain fragment-IL-12 second subunit. While these second generation immunoconjugates have some improved properties as compared to the first generation immunoglobulin-cytokine conjugates, development of more and even safer specific therapeutic agents is desirable for greater effectiveness against tumor cells and a decrease in the number and severity of the side effects of these products (e.g., toxicity, destruction of non-tumor cells, etc.). Additionally, it is desirable to identify ways to further stabilize immunoconjugates while maintaining acceptable therapeutic activity levels.
The present invention provides immunoconjugates that exhibit improved efficacy, high specificity of action, reduced toxicity, and improved stability in blood relative to known immunoconjugates.